It is well known that fibrous fillers such as glass fibers and carbon fibers are blended to improve the mechanical properties of a thermoplastic resin. One commonly used method of blending a fibrous filler is to melt-knead a thermoplastic resin and fiber chopped strands (short fibers) in an extruder.
In recent years, however, there has been an increased demand for higher-performance plastics, and rigidity comparable to those of metals has been demanded. To achieve rigidity comparable to those of metals, it is necessary to incorporate large amounts of fibrous filler while maintaining the fiber length long. Unfortunately, melt-kneading in an extruder, a commonly used method, has many problems such as reduction in flowability, reduction in mechanical properties due to fibrous filler breakage due to shearing during melt-kneading, and degradation of resins due to shear heating due to large amounts of fibrous filler. Melt-kneading a thermoplastic resin and a fibrous filler in a melt-kneader such as an extruder has a limit on the increase in performance.
As a resin composition that provides thin-wall molded articles with excellent appearance properties, mechanical properties, impact resistance, flowability, and moldability, there is proposed a glass-fiber reinforced polycarbonate resin composition made of an aromatic polycarbonate resin, an aromatic polycarbonate oligomer, a glass fiber including short fibers and long fibers, and a compounded-rubber-based graft copolymer (see, for example, JP 09-12858 A).
In addition, there are proposed, for example, a method (what is called “pultrusion”) in which continuous carbon fibers are impregnated with a matrix thermoplastic resin, molded, and cooled to produce a longitudinally bundled fiber-reinforced thermoplastic resin (see, for example, JP 04-153007 A), and a method in which a bundle of fibers impregnated with a resin, the fibers being selected from metal fibers, nonmetal fibers coated with metal, and carbon fibers, is formed with a forming nozzle at an outlet of a crosshead die and cut with a pelletizer to a predetermined length to produce a resin-impregnated fiber bundle in the form of pellets (see, for example, JP 2004-14990 A).
Furthermore, as a method of improving mechanical properties by leaving a fiber length long, there are proposed a method in which a long-fiber pellet and a short-fiber pellet are used in combination and a method in which a carbon-fiber chopped strand and a thermoplastic resin pellet are used in combination (see, for example, JP 2000-218711 A).
To multilayer a pellet, there are proposed a method in which a crystalline polyolefin and a flexible olefin copolymer are respectively used as a sheath and a core to reduce adhesion and improve handleability (see, for example, JP 2003-48991 A) and a method in which a multilayered pellet including a resin layer composed mainly of an ethylene/vinyl alcohol copolymer and a resin layer composed mainly of a polyamide is used to improve thermal stability, anti-retention properties, hot water resistance, and gas barrier properties (see, for example, JP 2009-242591 A).
The method disclosed in JP '858 improves properties such as flowability and surface appearance through the use of a short glass fiber but, unfortunately, results in poor mechanical properties.
Both of the methods disclosed in JP '007 and JP '990, in which a continuous fiber bundle is coated with a thermoplastic resin while being drawn through a die, have a problem of productivity such that the continuous fiber bundle tends to protrude from the thermoplastic resin coating at a high output rate.
The method disclosed in JP '711 can leave a fiber length long but, unfortunately, results in poor mechanical properties due to low fiber dispersibility.
The multilayered pellets according to the methods disclosed in JP '991 and JP '591 have improved handleability and productivity but, unfortunately, have poor mechanical properties.
It could therefore be helpful to provide a fiber-reinforced multilayered pellet that is excellent in productivity and flowability, provides molded articles with high mechanical properties, and allows for the incorporation of large amounts of fibrous filler.